LEWISTON, Maine — The rail authority that manages the Downeaster is working on a blueprint for its future, to be unveiled later this year, that includes bringing passenger rail service to Lewiston-Auburn.

Patricia Quinn, executive director of the Northern New England Passenger Rail Authority, is encouraging about the plan, even optimistic.

She’s also armed with caveats.

A whole lot has to happen first.

“I know the people of Lewiston-Auburn are anxious for service and our board has been anxious to get it there,” Quinn said in an interview Thursday.

Still in discussion are details such as costs and which rail lines would be used. Both Pan Am and state-owned St. Lawrence and Atlantic tracks come up this way from Portland.

“We have some thoughts on how it would work, but we don’t have anything definite yet,” Quinn said.

The rail authority holds the contract with Amtrak to operate the Downeaster train service from Brunswick to Boston, via Portland. Amtrak provides the equipment, crews, maintenance and ticket services.

The Brunswick to Portland line opened in November. The authority’s 2012 operating budget (before adding that leg): $15 million. Some $8 million came from revenue, $5.6 million from federal funds and $1.4 million from the state, Quinn said.

An average of 1,400 to 1,600 people ride the train each day, most leisure trippers, business travelers and students. About 300 people use it to regularly commute to work between Portland and Boston.

When it opened last fall, Quinn said the rail authority anticipated 100 people would ride the train north of Portland to Freeport and Brunswick each day. It’s ‘s been closer to 135. The train leaves the Brunswick station at 7:05 a.m. and 7 p.m. It arrives back in Brunswick at 12:30 p.m. and 8:20 p.m.

“We’re beating our target in ridership and revenue numbers,” Quinn said. “The response of the students has been phenomenal. I think the tourism piece to Freeport is a work in progress.”

This new blueprint, formally known as the Service Development Plan, will outline the rail authority’s vision for the next 20 years. Most rail projects move slowly. For that reason, it will likely include dollar estimates but not a firm timetable.

With a plan in hand, the rail authority — a quasi-state body created by the Legislature in 1995 — will pursue state and federal funds for the different projects.

“It took 12 years to start the Downeaster,” Quinn said. “It took another 11 to get to Freeport and Brunswick. We have experience and a great track record, if you can excuse the expression.”

She added, “If there’s a windfall, we can get it done soon. If not, it could take years.”

At least three things have to happen before train service is extended to the Twin Cities, Quinn said:

• Constructing a new layover facility in Brunswick, which is nearly underway;

• Building a new section of rail called the Portland “Y” track to give Brunswick trains a cleaner entry and exit to the Portland station;

• Adding 4 new miles of track in Yarmouth to allow cars to pass each other more easily.

Additionally, the Portland train station would need a center island platform with a set of train tracks on either side so a Lewiston-Auburn line would have something to pull up to, Quinn said.